This invention relates to reflow mounting of an electronic component on a mounting board and, more particularly, to a process and a mounting board for the reflow mounting.
FIGS. 7 and 8 illustrate one example of a conventional mounting board for mounting electronic components such as IC packages. In these figures, the mounting board comprises a substantially rectangular, electrically insulating substrate 1, a plurality of terminal electrodes 2 disposed on the major surfaces of the substrate 1 for external connection and a plurality of mounting lands 3 disposed on one of the major surfaces of the substrate 1 for mounting the electronic component. Although not illustrated, the terminal electrodes 2 and the mounting lands 3 are electrically connected to define an electrical circuit on the substrate 1. Each of the terminal electrodes 2 is coated with a solder layer 2a formed by the hot leveller process or the electrolyte plating method for the main purpose of protecting the terminal electrodes 2 against corrosion.
When it is desired to mount electronic components such as IC packages on the mounting lands 3 by the reflow process, each of the mounting lands 3 is coated with a printed solder paste layer 3a suitable for reflow process. The solder paste layer 3a has a melting point of about 183.degree. C. which is lower than a reflow temperature which is between 190.degree. C. and 210.degree. C. and which is almost equal to the melting point of the solder of the solder layer 2a on the terminal electrodes 2. The solder paste contains a solder of Sn 40% and Pb 60% with a small amount of flux.
Then, as illustrated in FIG. 10 the electronic components 4 to be mounted on the mounting board are placed on the solder layers 3a of the respective mounting lands 3 and the mounting board is placed on support pins 5a and 5b of a chain conveyor 5. It is seen from FIG. 10 that the support pins 5a are brought into contact with the protective solder 2a on the terminal electrodes 2 on the bottom side of the insulating substrate 1. The support pins 5b directly engage the bottom major surface of the insulating substrate 1.
As the mounting board on which the electronic components 4 are placed is passed through a reflow heating station and heated by a flow of hot air to about 190.degree. C. to 210.degree. C. the peak temperature, the solder layers 3a between the mounting lands 3 and the electronic components 4 melt and then are cooled to electrically and mechanically connect the electronic components 4 on the mounting lands 3 on the mounting board.
During this reflow heating process, the protective solder layers 2a on the terminal electrodes 2 also melt because the melting point of this solder 2a is substantially equal to that of the reflow solder 3a on the mounting lands 3 and is lower than the reflow temperature which usually is about 190.degree. C. to 210.degree. C., the peak temperature. Since the mounting board is supported by the support pins 5a and 5b of the chain conveyor 5, some of the support pins 5a are brought into contact with the molten solder 2a.
Therefore, the rate of the temperature rise rate and the cooling rate of the terminal electrode portion are very different from position to position and the molten solder 2a on the terminal electrodes 2 is collected and concentrated on a particular local position on the terminal electrodes 2 due to wetting. This causes the solidified solder layer 2a to have a very non-uniform thickness and irregular surface, whereby the mounting boards thus manufactured have a thickness different from one board to another and an ugly appearance which degrades the commercial value of the circuit boards. Accordingly, when the terminal electrodes 2 have such an irregular solder surface, the solder layers 2a on the terminal electrodes 2 had have to be corrected by hand.